As more particularly illustrated by FR-A-2 477 265, a boiler equipping a nuclear power station normally comprises a vertically positioned outer envelope, whose inner space is subdivided into two parts in the height direction by a horizontal plate known as a tube plate. The ends of the tubes of a bundle of inverted U-tubes are fixed to the tube plate and issue below the latter respectively into an admission collector or header and into a discharge collector or header for the water circulating in the primary circuit of the reactor and known as primary water. The water circulating in the secondary circuit of the reactor and known as secondary or feed water is injected into that part of the boiler positioned above the tube plate.
In the boiler described in FR-A-2 477 265, the feed water is injected by a semitoroidal, main supply collector, positioned above an annular recirculation space formed between the outer and inner envelopes covering the bundle of tubes and having its lower edge spaced from the tube plate.
The feed water introduced into the boiler by the main supply collector drops into the annular space and then rises between the tubes of the bundle within the inner envelope. The heat taken from the primary water circulating within the tubes then has the effect of evaporating the feed water when it enters the upper region of the lower envelope. The steam thus formed then traverses separators and driers, which lower the moisture content of the steam before the latter escapes to the turbines of the secondary circuit used for driving the electricity generators of the power station.
The condensation water held in the separators, generally referred to as recirculation water, descends again by gravity into the annular recirculation space, in order to again pass through the inner envelope of the generator.
The positioning of the main supply collector of the boiler above the annular recirculation space leads, as shown in FR-A-2 477 265, to the equipping of the collector with inverted J-shaped tubes, in order to prevent pressure jumps and surges, which might occur during the restarting of the pumps of the secondary circuit as a result of a pumping out of the supply collector. However, this procedure suffers form the disadvantage of complicating the manufacture of the boiler and consequently extending the manufacturing process and making it more expensive.
Moreover and as illustrated by U.S. Pat. Nos. 3,804,069, 3,896,770 and 3,916,843, consideration has also been given to supplying the feed water to a boiler by directly connecting a feed water admission pipe to a lower part of the inner envelope, so as to make the water penetrate directly at the bottom of the cold branches of the tubes of the bundle. Deflectors positioned facing the admission pipe and around the cold branches then form a device for preheating the feed water circulating between the tubes.
Even though the solution described in the latter documents avoids the disadvantages caused by the installation of the main supply collector in the upper part, it suffers from the disadvantage of the subjecting the lower parts of the cold branches of the tubes to significant transverse flows and of not permitting controlled distribution of the feed water flow rate over the boiler cross-section.
Moreover, if migrating bodies such as welding rods, screws, bolts, etc., inadvertently introduced into the secondary circuit during their manufacture, enter the steam generator by the admission pipe for the feed water, they can jam between the tubes of the bundle and damage the tubes.
Moreover, the existing boilers in which the introduction of the feed water takes place in the bottom of the secondary part do not make it possible to carry out a controlled distribution of the feed water and the recirculation water on the periphery of the boiler, whereas such a controlled distribution would be desirable in order to bring about an optimum reconciliation between the contradictory requirements, among which reference is mainly made to:
obtaining maximum efficiency of the steam generator in normal operation; PA1 the need to ensure a satisfactory operation of the steam generator during an incident requiring the use of a standby or emergency supply collector, generally positioned above the annular recirculation space; and PA1 reducing to a minimum the thermal stresses suffered by the outer envelope and the tube sheet of the boiler. PA1 a vertically axed, outer envelope, PA1 a horizontal tube plate tightly fixed within the outer envelope, PA1 a bundle of inverted U-tubes having hot branches and cold branches and each having two ends fixed to the tube plate and issuing below the latter, respectively into an admission collector and into a discharge collector for the primary fluid, PA1 an inner envelope covering the tube bundle and having a lower edge spaced from the tube plate and forming with the outer envelope an annular recirculation space, PA1 secondary water supply means, and PA1 means for the separation of recirculation water able to drop again through the recirculation space and secondary steam which can be extracted from the boiler, the separating means being positioned above the inner envelope. The secondary water supply means comprises at least one supply pipe issuing directly into the annular recirculation space, at least one perforated collar being placed in the annular recirculation space at a higher level than that of the supply pipe, so as to oppose any raising of the secondary water into this space and so as to ensure controlled circumferential distribution of the recirculation water descending into the space.